drivers: timer: nrf: Add configurable clock startup mode

Added configuration for approach to starting system clock source.
There are 3 options: no wait, wait untill available, wait until
stable.

Added support for those modes in clock control driver which handles
low frequency source clock.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>

drivers/clock_control/clock_control_nrf.c

@@ -444,6 +444,7 @@ static void lfclk_spinwait(nrf_clock_lfclk_t t)
 
 	while (!(nrfx_clock_is_running(d, (void *)&type) && (type == t))) {
 		/* empty */
+		/* k_cpu_idle(); */
 	}
 }
 
@@ -463,16 +464,21 @@ void z_nrf_clock_control_lf_on(enum nrf_lfclk_start_mode start_mode)
 		__ASSERT_NO_MSG(err >= 0);
 	}
 
+	/* In case of simulated board leave immediately. */
+	if (IS_ENABLED(CONFIG_BOARD_NRF52_BSIM)) {
+		return;
+	}
+
 	switch (start_mode) {
-	case NRF_LFCLK_START_MODE_SPINWAIT_STABLE:
+	case CLOCK_CONTROL_NRF_LF_START_STABLE:
 		lfclk_spinwait(CLOCK_CONTROL_NRF_K32SRC);
 		break;
 
-	case NRF_LFCLK_START_MODE_SPINWAIT_RUNNING:
+	case CLOCK_CONTROL_NRF_LF_START_AVAILABLE:
 		lfclk_spinwait(NRF_CLOCK_LFCLK_RC);
 		break;
 
-	case NRF_LFCLK_START_MODE_NOWAIT:
+	case CLOCK_CONTROL_NRF_LF_START_NOWAIT:
 		break;
 
 	default:

drivers/counter/counter_nrfx_rtc.c

@@ -534,7 +534,7 @@ static int init_rtc(const struct device *dev, uint32_t prescaler)
 	NRF_RTC_Type *rtc = nrfx_config->rtc;
 	int err;
 
-	z_nrf_clock_control_lf_on(NRF_LFCLK_START_MODE_NOWAIT);
+	z_nrf_clock_control_lf_on(CLOCK_CONTROL_NRF_LF_START_NOWAIT);
 
 	nrf_rtc_prescaler_set(rtc, prescaler);
 

drivers/timer/Kconfig

@@ -155,6 +155,42 @@ config NRF_RTC_TIMER
 	  Counter NRF_RTC1 and provides the standard "system clock driver"
 	  interfaces.
 
+if NRF_RTC_TIMER
+
+choice
+	prompt "Clock startup policy"
+	default SYSTEM_CLOCK_WAIT_FOR_STABILITY
+
+config SYSTEM_CLOCK_NO_WAIT
+	bool "No wait"
+	help
+	  System clock source is initiated but does not wait for clock readiness.
+	  When this option is picked, system clock may not be ready when code relying
+	  on kernel API is executed. Requested timeouts will be prolonged by the
+	  remaining startup time.
+
+config SYSTEM_CLOCK_WAIT_FOR_AVAILABILITY
+	bool "Wait for availability"
+	help
+	  System clock source initialization waits until clock is available. In some
+	  systems, clock initially runs from less accurate source which has faster
+	  startup time and then seamlessly switches to the target clock source when
+	  it is ready. When this option is picked, system clock is available after
+	  system clock driver initialization but it may be less accurate. Option is
+	  equivalent to waiting for stability if clock source does not have
+	  intermediate state.
+
+config SYSTEM_CLOCK_WAIT_FOR_STABILITY
+	bool "Wait for stability"
+	help
+	  System clock source initialization waits until clock is stable. When this
+	  option is picked, system clock is available and stable after system clock
+	  driver initialization.
+
+endchoice
+
+endif # NRF_RTC_TIMER
+
 source "drivers/timer/Kconfig.stm32_lptim"
 
 config RISCV_MACHINE_TIMER

drivers/timer/nrf_rtc_timer.c

@@ -189,6 +189,12 @@ void rtc_nrf_isr(const void *arg)
 int z_clock_driver_init(const struct device *device)
 {
 	ARG_UNUSED(device);
+	static const enum nrf_lfclk_start_mode mode =
+		IS_ENABLED(CONFIG_SYSTEM_CLOCK_NO_WAIT) ?
+			CLOCK_CONTROL_NRF_LF_START_NOWAIT :
+			(IS_ENABLED(CONFIG_SYSTEM_CLOCK_WAIT_FOR_AVAILABILITY) ?
+			CLOCK_CONTROL_NRF_LF_START_AVAILABLE :
+			CLOCK_CONTROL_NRF_LF_START_STABLE);
 
 	/* TODO: replace with counter driver to access RTC */
 	nrf_rtc_prescaler_set(RTC, 0);
@@ -206,7 +212,7 @@ int z_clock_driver_init(const struct device *device)
 		set_comparator(counter() + CYC_PER_TICK);
 	}
 
-	z_nrf_clock_control_lf_on(NRF_LFCLK_START_MODE_NOWAIT);
+	z_nrf_clock_control_lf_on(mode);
 
 	return 0;
 }

include/drivers/clock_control/nrf_clock_control.h

@@ -37,9 +37,9 @@ enum clock_control_nrf_type {
 
 /** @brief LF clock start modes. */
 enum nrf_lfclk_start_mode {
-	NRF_LFCLK_START_MODE_NOWAIT,
-	NRF_LFCLK_START_MODE_SPINWAIT_RUNNING,
-	NRF_LFCLK_START_MODE_SPINWAIT_STABLE,
+	CLOCK_CONTROL_NRF_LF_START_NOWAIT,
+	CLOCK_CONTROL_NRF_LF_START_AVAILABLE,
+	CLOCK_CONTROL_NRF_LF_START_STABLE,
 };
 
 /* Define 32KHz clock source */